How i can check real price my domain for free

first sorry for my english too bad i'am live in france.
I Need to know about my domain's real price,
me and my partner have create a platform expired domain and differents stuff about domain's with really potential for selling it, my question are: How for free i can check real price for all my domain's please ?
I have only .fr and .com domain and one in .net
Same if is approxymatively for 50/100$ but 85% chance give real price i'am appreciate it.
Thank you very much for all answer's

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